solid oxide fuel cells
TRANSCRIPT
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WELCOME ALL!!!!
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FUEL CELL
An upgraded version of Batteries
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WHAT IS A FUEL CELL?
A fuel cell is an electrochemical energy conversion device that converts hydrogen and oxygen into electricity, heat, and water as a result of a chemical reaction.
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WHAT DOES A FUEL CELL ACTUALLY CONTAINS?
1. CATHODE- the positive electrode2. ANODE- the negative electrode3. ELECTROLYTE- in which the reactions
take place4. AN INTERCONNECT(in case of a stack)-
for electron transfer5. SEALS- To act as barrier between
components
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SALIENT FEATURES
Highly efficient electric power generation system (can be as high as 70-80%)
Effective utilization high temperature waste heat
Environmental friendly power generation
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WHETHER A SINGLE FUEL CELL IS ENOUGH ? A single fuel cell generates a tiny
amount of direct current (DC) electricity. In practice, many fuel cells are usually assembled into a stack. Cell or stack, the principles
are the same.
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REQUIREMENTS OF ELECTROLYTE
Ionically conductive -oxygen ion transport
Chemically stable (at high temperatures as well as in reducing and oxidizing environments)
Gas tight/free of porosity Uniformly thin layer (to minimize ohmic
losses)
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ELECTROLYTEMost widely used electrolyte is Yttrium
doped zirconium oxide (YSZ) Advantages of YSZi. ionic conductivityii. chemical stabilityiii. mechanical strength Disadvantages of YSZi. low ionic conductivity
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ELECTROLYTE SolutionI. Decrease the thickness of the YSZ
electrolyte II. Find other materials to replace the
yttrium like Scandium-doped zirconium oxide has higher conductivity than YSZ but high cost of scandium is a disadvantage
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CATHODE-REQUIREMENTS
High electronic conductivity Chemically compatible with neighboring
cell component (usually the electrolyte) Should be porous Stable in an oxidizing environment Large triple phase boundary Catalyze the dissociation of oxygen Adhesion to electrolyte surface
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CATHODELanthanum strontium manganite(LSM) is the cathode
Advantages of LSM
I. Compatibility with doped zirconia electrolytes
II. Similar coefficient of expansion to YSZ and thus limits stresses
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CATHODE Disadvantages of LSMI. LSM is a poor ionic conductor, and so
the electrochemically active reaction is limited to the triple phase boundary (TPB) where the electrolyte, air and electrode meet.
II. LSM works well as a cathode at high temperatures, but its performance quickly falls as the operating temperature is lowered below 800 °C.
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ANODE-REQUIREMENTS
Electrically conductive High electro-catalytic activity Large triple phase boundary Stable in a reducing environment Can be made thin enough to avoid mass transfer
losses, but thick enough to provide area and distribute current
Thermal expansion coefficient similar neighboring cell component
Chemically compatible with neighboring cell component
Fine particle size
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ANODE Ceramic anode layer must be very
porous to allow the fuel to flow towards the electrolyte
The most common material used is a cermet made up of nickel mixed with the ceramic material that is used for the electrolyte
The anode is commonly the thickest and strongest layer
The anode’s job is to use the oxygen ions that diffuse through the electrolyte to oxidize the hydrogen fuel
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ANODEADDITIONAL USES:
Function of the anode is to act as a catalyst for steam reforming the fuel into hydrogen.
This provides another operational benefit to the fuel cell stack because the reforming reaction is endothermic, which cools the stack internally.
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INTERCONNECT-REQUIREMENTS
Stable under high temperature oxidizing and reducing environments
Very high electrical conductivity High density with “no open porosity” Strong and high creep resistances for planar
configurations Good thermal conductivity Phase stability under temperature range Resistant to sulfur poisoning, oxidation and
carburization Low materials and fabrication cost
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INTERCONNECT The interconnect can be either a metallic or
ceramic layer that sits between each individual cell.
It connects each cell in series, so that the electricity each cell generates can be combined.
a metallic 95Cr-5 Fe alloy is the most commonly used interconnect
Ceramic materials are also under considerations
DRAWBACKS: these ceramic interconnect materials are very
expensive as compared to metals.
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SEALS-REQUIREMENTS
Electrically insulating Thermal expansion compatibility with
other cell components Chemically and physically stable at high
temperatures Gastight Chemically compatible with other
components Provide high mechanical bonding strength Low cost
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HOW DO FUEL CELLS WORK? PURPOSE produce an electrical current that can
be directed outside the cell to do work GENERAL WORKING OF FUEL CELLS1. Hydrogen atoms enter a fuel cell at
the anode 2. Hydrogen atoms are now ionized, and
carry a positive electrical charge.3. Negatively charged electrons provide
the current through wires to do work.
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In some cells, Oxygen enters the fuel cell at the cathode and combines with the electrons and hydrogen.
In other cell types the oxygen picks up electrons and then combines with hydrogen
Electrolyte must permit only the appropriate ions to pass between the anode and cathode
Fuel cells create electricity chemically. Therefore, fuel cells are more efficient in extracting energy from a fuel
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TYPES Alkali fuel cells Molten Carbonate fuel cells (MCFC) Phosphoric Acid fuel cells (PAFC) Proton Exchange Membrane (PEM) fuel
cells Solid Oxide fuel cells (SOFC)
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CHARACTERISTICS OF SOFC Fuel cells can continuously make
electricity if they have a constant fuel supply.
SOFCs that operate at higher temperatures -- between about 1100 and 1800 degrees Fahrenheit
Can run on a wide variety of fuels, including natural gas, biogas, hydrogen and liquid fuels such as diesel and gasoline
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CHARACTERISTICS OF SOFC Each SOFC is made of ceramic
materials, which form three layers: the anode, the cathode and the electrolyte
The big advantage to fuel cells is that they're more efficient than traditional power generation
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SOLID OXIDE FUEL CELL
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GENERAL WORKING
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WORKING PRINCIPLE
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SOFC – WORKING PRINCIPLE SOFC essentially consists of two
porous electrodes separated by a dense, oxide ion conducting electrolyte.
Oxygen supplied at the cathode (air electrode) reacts with incoming electrons from the external circuit to form oxide ions
These ions migrate to the anode (fuel electrode) through the oxide ion conducting electrolyte.
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SOFC – WORKING PRINCIPLE At the anode, oxide ions combine with
hydrogen (and/or carbon monoxide) in the fuel to form water (and/or carbon dioxide), liberating electrons.
Electrons (electricity) flow from the anode through the external circuit to the cathode.
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SOLID OXIDE FUEL CELLSADVANTAGES high efficiency, long-term stability, fuel flexibility, low emissions, and relatively low cost. DISADVANTAGES high operating temperature which
results in longer start-up times and mechanical and chemical compatibility issues.
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APPLICATIONS SOFC are being targeted for use in power and heat
generation for homes and businesses as well as auxiliary power units for electrical systems in vehicles.
SOFC also can be linked with a gas turbine, in which
the hot, high pressure exhaust of the fuel cell can be
used to spin the turbine, generating a second source of
electricity. Using planar SOFCs, stationary power generation
systems of from 1-kW to 25-kW size have been fabricated and tested by several organizations
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APPLICATIONS Rolls-Royce Fuel Cell Systems Ltd is
developing a SOFC gas turbine hybrid system fueled by natural gas for power generation applications on the order of a megawatt (e.g. Futuregen).
Ceres Power Ltd. has developed a low cost and low temperature (500–600 degrees) SOFC stack using cerium gadolinium oxide (CGO) in place of current industry standard ceramic, yttria stabilized zirconia (YSZ), which allows the use of stainless steel to support the ceramic.
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APPLICATIONS Solid Cell Inc. has developed a unique, low cost cell
architecture that combines properties of planar and tubular designs, along with a Cr-free cermet interconnect.
The high temperature electrochemistry center (HITEC) at the University of Florida, Gainesville is focused on studying ionic transport, electro catalytic phenomena and micro structural characterization of ion conducting materials.
SiEnergy Systems, a Harvard spin-off company, has demonstrated the first macro-scale thin-film solid-oxide fuel cell that can operate at 500 degrees.
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APPLICATIONS Delphi Automotive Systems are
developing an SOFC that will power auxiliary units in automobiles and tractor-trailers
Research is also going on in reducing start-up time to be able to implement SOFCs in mobile applications
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ADVANCEMENTS
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ADVANCEMENTS
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ADVANCEMENTS
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